Injection molding is one of the dominant forms of manufacture of plastic articles in the world today. However, a problem with uniformity of product plagues this process of injecting a quantity, or shot, of melted plastic into a mold. Uniformity is sacrificed due to the inability to perfectly control the quantity of material injected into the mold. This imperfection is due primarily to failure of a non-return valve, found on most injection molding machines, to close in a consistent, repeatable manner and then properly seal against back flow of material during the injection step.
A review of the prior art illustrates two primary methods of sealing against this back flow of material during the injection step: a ring-type shut off valve or a ball-type check valve. With either method, as the injection ram strokes forward, a ball or piston is forced against a seat or a tapered ring is forced against another ring with a complementary taper. There are various alterations on these devices that either control the length of the stroke or the shape of the valve parts and the plastic flow passages. However, in either case, plastic leakage flow over the ball or piston, or under the ring, creates a pressure drop across this moving member of the valve. This pressure drop becomes the primary force to close the valve. Any variation in the leakage flow before the valve closes results in a variation in the quantity of plastic trapped in front of the closed valve. Leakage flow variations occur with these valves resulting in product variations of major or minor significance depending on the product being made, operating conditions, and plastic material characteristics.
The sealing surfaces in either a ring-type or ball-type valve can become contaminated with particles which prevent a perfect seal. This allows for material migration back through the valve rather than forward into the mold. Variations in this lost quantity of material will cause an imperfect product from the mold which may be unacceptable for its intended use. Detection of these unacceptable products requires significant inspection costs or inconvenience for the ultimate user of the product. To improve quality, many manufacturers have implemented statistical process control (SPC) which attempts to define and control process variables so that all supplied product is sufficiently identical, eliminating the need for inspection.
Therefore, a need exists for a non-return valve which never fails to furnish the same shot size regardless of plastic, fillers, contamination, product produced, or wear. This valve should be designed to allow its incorporation into existing injection molding machines or any other device which utilizes a non-return valve. This valve should not be dependent on leakage through the valve to generate the force necessary to move the valve to its closed position. Furthermore, this valve should be designed so that the seal can never be impaired by particles. Thus, each time the valve seals, the sealing action should shear and clear away any such particles.